1. Field of the Invention
This invention relates to a toner used in an image forming process that utilizes electrophotography or electrostatic recording. More particularly, it relates to an image forming process making use of a toner having i) toner base particles containing a binder resin, a colorant and a release agent and ii) an external additive powder.
2. Description of the Related Art
A developing process used in electrophotographic apparatus or the like is a process in which a toner is made to adhere to an electrostatic latent image formed on an electrostatic latent image bearing member to render the electrostatic latent image visible to form a toner image, which is then transferred to a recording medium, followed by a fixing step to obtain a fixed image.
As developers used in electrophotography, they are grouped into a one-component developer and a two-component developer. They are also grouped into magnetic and non-magnetic, in respect of toners.
The toner of the present invention may be used in either developer of the both.
In recent years, in electrophotographic systems, a one-component developing system has come into wide use. In such a system, the toner is electrostatically charged by rubbing friction between a toner carrying member and a toner layer thickness control member. Employment of such a system of charging the toner electrostatically has made it easy to succeed in making printers low-price and compact. With a demand for color image formation in recent years, a non-magnetic one-component developing system has also come into wide use, which is advantageous to printers being made low-price and compact.
In respect of making the toner stably chargeable, the two-component developer is advantageous. Accordingly, both the one-component and two-component developers and developing systems are employed in electrophotography.
Meanwhile, as printers are being made low-price and compact, it is also sought to make total powder consumption small. In particular, in the fixing step, it is sought to lessen the energy to be consumed. For this end, it is necessary to make a toner more advanced which is fixable at a lower temperature.
As the toner is thus being made fixable at a lower temperature, the toner shows a tendency to have a poor durability to mechanical stress. This is considered due to the fact that giving preference to the fixing performance of toner in order to make a toner adaptable to more low-temperature fixing makes the toner have poor properties to mechanical stress in the range of service temperature set usually.
Further, there is in the market a high demand for printers to be made more high-speed. Problems on printing at a high speed are not only a demand for the above low-temperature fixing performance but also a demand for much higher resistance to stress. In general, high-speed machines have a fast mechanical movement in the apparatus. Hence, heat tends to be generated at their shafts of rollers or the like and rubbing portions. In particular, in the one-component developing system, in which the toner is electrostatically charged by the rubbing friction between a toner carrying member and a toner layer thickness control member, it is demanded at a technically high level to balance the demand for toner and the durability of toner that come as printers are made high-speed.
In order to meet such demands at a high level, toners produced by polymerization, toners obtained by making pulverization toners spherical by heat, toners obtained by agglomerating emulsification particles, and so forth are proposed as toner particles.
Many of such toners are those designed to have a higher mechanical strength than any conventional toners of a melt-kneading and pulverization type. For example, resins that compose toner particle surfaces and toner particle interiors are compositionally changed to provide toner particles with a core-shell structure. With such a structure, a toner is proposed in which a material having a high mechanical strength is used in the shell and a material effective in fixing is used in the core (see, e.g., Japanese Patent No. 3055119).
However, even with use of such a toner, faulty toner transport accompanied by a phenomenon that the surface of a toner transport member is contaminated with toner (i.e., toner melt adhesion to a toner carrying member) tends to occur in the non-magnetic one-component developing system having a high process speed.
As a result of examination of such toner melt adhesion to a toner carrying member, it is considered to be caused by particles having small particle size (a fine-powder component) in toner which do not participate in development and are accumulated on the toner carrying member surface to become thickened thereon during running (extensive operation). In order to lessen such option of particles in the toner during running, it is important to make the toner highly releasable from the toner carrying member surface and to make the toner stably chargeable.
Such a toner also tends to bring about problems such as any image fog and any line image development (development lines) which is caused by restriction due to adhesion of a toner to the toner layer thickness control member.
It is preferable that the toner improved in mechanical strength as stated above is used in such a non-magnetic one-component developing image forming apparatus having a high process speed. However, anything has not been obtained which can meet demands for further energy saving and higher speed.
Meanwhile, in regard to improvement in running performance of toners, various surface treating agents (external additives) have been proposed.
Of these proposals, it has been invented to use a fatty acid metal salt as an external additive of toner base particles.
For example, a toner is disclosed which is composed of negatively chargeable toner base particles and a fatty acid metal salt (see, e.g., Japanese Patent Application Laid-open Nos. H08-129304 and 2002-014488).
A toner is also disclosed which contains a fatty acid metal salt having a specific volume average particle diameter (see, e.g., Japanese Patent Application Laid-open Nos. 2004-163807 and 2002-296829).
A toner is still also disclosed which has specified the circularity of a toner containing a fatty acid metal salt and the ratio of particle diameters of the fatty acid metal salt (see, e.g., Japanese Patent Application Laid-open No. 2006-017934).
Toners making use of these external additives are effective in improving blade cleaning performance, in improving blade turn-up resistance and in keeping the electrostatic latent image bearing member surface from abrading. They may also be effective in achieving uniform chargeability and in keeping drum filming from occurring.
Such toners show superior lubricity on the electrostatic latent image bearing member surface. However, the fatty acid metal salt tends to be selectively consumed during running, so that the toners may come less effective in keeping such lubricity at the latter half of running. In addition, in order for the toners to be effective as stated above, the fatty acid metal salt must be added in a sufficient quantity, but this tends to bring about problems such as charging-part contamination under the influence of any excess fatty acid metal salt. Further, such a problem tends to be a remarkable problem in color toners used in image forming apparatus of a high-speed type, and hence, when used in the apparatus of a high-speed type, it is necessary to keep use of a sufficiently effective material in a proper quantity.
Invention has also been made on a toner, taking note of the relationship between its number average particle diameter, the proportion of 3.17 μm or smaller particles and the number average particle diameter of a fatty acid metal salt.
According to such invention, a toner can be obtained which may less wear a photosensitive member, can be free of any image fog or blurred images, can form uniform halftone images and also may less cause toner spots around minute dots (see, e.g., Japanese Patent No. 3467966).
A toner is further disclosed which is a one-component developer containing toner base particles having a melt viscosity within a specific range, a fluidity improver and a fatty acid metal salt, and has specified the relationship between toner particle diameter and fatty acid metal salt particle diameter. According to this, a toner is obtained which promises a high image density, may less cause fog and can provide a superior sharpness (see, e.g., Japanese Patent Application Laid-open No. H09-311499).
The toner as noted above may less cause fog and enables reproduction of minute images. However, further improvement is necessary for its application to the non-magnetic one-component developing image forming apparatus having a high process speed. Such invention has not succeeded in achievement of any sufficient performance.
Thus, the techniques conventionally available as stated above have some subjects to be settled for high demands made at present.
Meanwhile, as typical methods presently available for producing the fatty acid metal salt may include a method in which a solution of an inorganic metal compound is dropwise added to a solution of an alkali metal salt of a fatty acid to carry out reaction (a double decomposition process), and a method in which a fatty acid and an inorganic metal compound are kneaded at a high temperature to carry out reaction (a melting process).
Further, various studies are made on how the fatty acid metal salt be made finer. For example, invention has been made on a production method by which the fatty acid metal salt is made into fine particles, and on a toner making use of such particles (see, e.g., Japanese Patent No. 3906580).
Such a toner is one in which the fatty acid metal salt is made finer by controlling solvent concentration and temperature at the time of synthesis when it is produced by a wet process. The toner containing such a fatty acid metal salt well brings out the performance as a lubricant at least and can be highly effective as a cleaning aid.